This invention relates to radio receivers capable of operating in either an FM or an AM mode and having an automatic noise muting feature in the FM operating mode.
In FM reception, such muting has been known as squelching, interstation-noise suppression or quiet automatic-volume control. It's purpose is to shut off or attenuate the audible noise that is heard when either the station to which the receiver is tuned ceases to transmit or when scan tuning the receiver between transmitting stations whether manually or by automatic means. The noise is generated in the receiver itself in the antenna and in the atmosphere. When alone it is normally demodulated by the FM detector and will, unless muted or blocked, produce disquieting audible noise. When the receiver is tuned to a strong radio signal, the receiver goes into limiting and the noise is preempted by the signal.
Generally, muting has been accomplished by either one of two ways, (a) by unblocking the audio channel in response to a large signal or (b) by blocking the audio channel in response to the presence of noise from the IF amplifier which indicates the absence of radio signal limiting. In some radio designs both are employed simultaneously.
The second means (b) can provide a more reliable measure of the limiting that is effected only by the presence of the desired (tuned-in) radio signal, because it admits of the use of a high quality band pass filter to eliminate signal frequencies outside this band that may be falsely recognized as noise. Thus the second means (b) generally imbues a radio with superior performance. However, such high quality filters must employ discrete components and thus represent a high cost factor in a radio that is for the most part composed of one or more integrated circuits.
It is therefore an object of this invention to provide a high performance FM noise-activated mute in a radio capable of operation in an FM or an AM reception mode.
It is a further object of this invention to provide such a radio comprising integrated circuits wherein high performance is obtained without a high-quality discrete-components filter leading to low manufacturing costs .